Ensure time zone data gets installed on Ubuntu

The "tzdata" Ubuntu package is no longer a dependency of any of the
packages we install on that system.  Make sure it always gets installed
by explicitly including it in the list of packages to install on
Debian/Ubuntu, so that the "time_test" BIND unit test can pass.

docker/bind9/debian-template/Dockerfile

@@ -73,6 +73,7 @@ RUN apt-get -y install		\
 	python3-pytest		\
 	python3-requests	\
 	sudo			\
+	tzdata			\
 	xsltproc		\
 	zip			\
 	zlib1g-dev